.\"	$NetBSD: alloca.3,v 1.16 2012/10/24 22:56:27 wiz Exp $
.\"
.\" Copyright (c) 1980, 1991, 1993
.\"	The Regents of the University of California.  All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\"    notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\"    notice, this list of conditions and the following disclaimer in the
.\"    documentation and/or other materials provided with the distribution.
.\" 3. Neither the name of the University nor the names of its contributors
.\"    may be used to endorse or promote products derived from this software
.\"    without specific prior written permission.
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
.\" ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
.\" SUCH DAMAGE.
.\"
.\"     from: @(#)alloca.3	8.1 (Berkeley) 6/4/93
.\"
.Dd October 24, 2012
.Dt ALLOCA 3
.Os
.Sh NAME
.Nm alloca
.Nd memory allocator
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In stdlib.h
.Ft void *
.Fn alloca "size_t size"
.Sh DESCRIPTION
The
.Fn alloca
function
allocates
.Fa size
bytes of space in the stack frame of the caller.
This temporary space is automatically freed on
return.
.Sh RETURN VALUES
The
.Fn alloca
function returns a pointer to the beginning of the allocated space.
If the allocation failed, a
.Dv NULL
pointer is returned.
.Sh SEE ALSO
.Xr cc 1 ,
.Xr brk 2 ,
.Xr calloc 3 ,
.Xr getpagesize 3 ,
.Xr malloc 3 ,
.Xr realloc 3 ,
.Xr security 7
.Sh CAVEATS
Few limitations can be mentioned:
.Bl -bullet
.It
The
.Fn alloca
function is not part of any C standard and its use is not portable.
.It
The
.Fn alloca
function should be supplied by the compiler because the compiler is allowed to
make assumptions about the stack and frame pointers.
The libc
.Fn alloca
implementation cannot account for those assumptions.
While there is a
machine dependent implementation of
.Fn alloca
in libc, its use is discouraged and in most cases it will not work.
Using this implementation will produce linker warnings.
.It
The
.Fn alloca
function is unsafe because it cannot ensure that the pointer
returned points to a valid and usable block of memory.
The allocation made may exceed the bounds of the stack, or even go
further into other objects in memory, and
.Fn alloca
cannot determine such an error.
For that all
.Fn alloca
allocations should be bounded and limited to a small size.
.It
Since
.Fn alloca
modifies the stack at runtime and the stack usage of each function frame
cannot be predicted, it makes many compiler security features
(such as
.Xr cc 1
.Fl fstack-protector )
useless for the calling function.
See
.Xr security 7
for a discussion.
.El
.\" .Sh HISTORY
.\" The
.\" .Fn alloca
.\" function appeared in
.\" .Bx ?? .
.\" The function appeared in 32v, pwb and pwb.2 and in 3bsd 4bsd
.\" The first man page (or link to a man page that I can find at the
.\" moment is 4.3...
